There is a large demand to develop miniaturized blood analysis systems for rapid and reliable point-of-care testing and monitoring. Such systems would significantly enhance the quality of healthcare by offering immediate measurement of several clinically relevant parameters that can be used to assess a patient's health.
Microfluidic devices fabricated using MEMS technology offer the possibility of delivering such a system. In order to realize disposable biochips (to avoid cross-contamination of samples and measurement errors) it is necessary to have a reliable microfluidic control system that can be implemented on a low-cost substrate. The use of MEMS based devices such as microvalves and micropumps adds to the cost of the system and necessitates the use of complex control systems. Another challenging factor in realizing disposable biochips is a simple yet non-complex actuation source for fluidic driving. Micropumps are typically expensive and also difficult to integrate. The sampling of blood is considered another challenging aspect since most microfabricated needles cannot puncture to sufficient depth for venous blood sampling, which is the most commonly accepted sample source.
Sensitive biosensor arrays are also desired which can reliably measure the clinically relevant parameters at the typical concentrations observed in human blood with adequate accuracy and precision. Furthermore, the sensor array should be fully integrated with the biochip, preferably on the same low-cost substrate as the remaining biochip for ease of integration and low-cost. The developed sensor array should be able to perform well outside laboratory environments where interfering signals such as temperature variations must not affect the sensor performance.
To realize a fully integrated, disposable biochip that can be deployed for point-of-care testing (POCT) applications, a number of criteria have to be met. Based in part on the discussion above, some of these can be listed as: (a) low cost per disposable biochip wherein the low cost necessitates the use of low-cost biocompatible substrate materials; (b) a low cost, mass producible fabrication process for the biochips wherein the high manufacturing volumes reduce the per device cost; (c) fully-integrated sampling capability, the use of which allows for the biochip to directly acquire blood samples; (d) on-chip storage of reagents/buffers such that the biochip is a self-contained unit; (e) integrated biosensor array wherein the biosensor array is fabricated on essentially the same substrate as the rest of the biochip and can be easily integrated with the rest of the biochip; (f) a low-cost, high volume compatible fabrication process for the biosensor array wherein the high volume manufacturing drives down the cost of the each sensor array; (g) a fully integrated pumping modality which can achieve the desired microfluidic sequencing wherein the pumping technique does not involve any moving parts to increase the reliability of operation; (h) the fully integrated pumping modality which requires minimal external power for operation; (i) the pumping modality wherein the micropump can be easily integrated with the biochip and whose operation can be regulated with minimal control signals (j) preferably passive microfluidic control system with no moving parts for flow sequence regulation; (k) the fully integrated biochip wherein minimal control signals are required to initiate and maintain microfluidic sequencing and interrogate the biosensor arrays; and (l) a suitable analyzer module (handheld or smaller) which can be used to regulate operation of the biochip.
To date, a number of inventions have described a so-called “integrated biochip” or “disposable biochip”. For example, WO9960397A1, incorporated in its entirety by reference herein, describes a liquid analysis cartridge wherein the cartridge is fabricated on a low-cost substrate using lamination techniques and incorporates most of the requirements listed above with the notable exception of an on-chip pumping source. WO0021659A1, incorporated herein in its entirety by reference, describes yet another microfluidic device wherein the device is manufactured using a specialized ceramic substrate and incorporates only the microfluidic components described above. Furthermore, this device also uses active power-hungry components thereby rendering it less than ideal for POCT applications. WO9933559A1, incorporated herein in its entirety by reference, describes a fairly complex microfluidic device that relies on re-constitution of on-chip stored reagents and also has on-chip reaction chambers for biochemical reactions. This system too, does not have on-chip pumping sources. U.S. Pat. No. 566,093, incorporated herein in its entirety by reference, describes a “Disposable device in Diagnostic Assays” wherein passive capillary fluidic driving and valving is used for flow regulation. This system is more suited for POCT applications, although it does not contain any integrated sampling scheme. Other examples of diagnostics microfluidic devices are presented in U.S. Pat. No. 6,537,501, US2002015586A1, U.S. Pat. No. 647,927, U.S. Pat. No. 5,405,510, US20040115094A1, U.S. Pat. No. 6,773,671, and US20030186228A1, all incorporated herein in their entirety by reference. An interesting microfluidic device which uses centrifugal forces (generated by a spinning motion imparted to the substrate) is disclosed in WO03080868A1, incorporated herein in its entirety by reference. All these system suffer from a couple of notable drawbacks namely; the lack of an efficient, low-power consumption pumping arrangement and lack of an integrated sampling arrangement. EP1245943A2, incorporated herein in its entirety by reference, describes a system for collecting whole blood for analysis by a microfluidic system and as clearly illustrated herein, the device is fairly complex with a filtering scheme together with the sampling canula. EP1245279A2, incorporated herein in its entirety by reference, illustrates the most commonly used approach for delivering samples to the microfluidic system, wherein the sample is first collected using a conventional syringe and subsequently transferred to the microdevice using an appropriate interface. This inhibits the widespread use of the microfluidic devices, partly due to the need for an extra sampling and also because this method employs a conventional needle, which is relatively painful when compared with microneedles. However, as mentioned previously, most microneedles fabricated using MEMS techniques are not suitable for venous sampling due to the either dimensional or structural integrity concerns.
A notable exception is the handheld analysis system developed by the I-STAT® Corporation which also uses a disposable analysis cartridge. The cartridge is not manufactured using conventional MEMS based processes but the dimensions are fairly small compared to the conventional analysis equipment thereby making it a potentially viable candidate for competing in the POCT area. The biosensors for the aforementioned analysis cartridge are manufactured on a Silicon substrate whereas the bulk of the cartridge itself is manufactured from. plastics as illustrated in U.S. Pat. No. 5,837,454 and U.S. Pat. No. 6,306,594, incorporated herein in their entirety by reference. Coupled with the high intrinsic cost of the Silicon substrate, the complex, time-consuming assembly for dissimilar substrates further adds to the cost of the device. Also, this device uses an external motor to actuate a plunger which subsequently causes fluidic displacement. Consequently, the energy demand for operating this cartridge is fairly high. Finally, this device too requires sample injection from a syringe wherein the blood has been previously collected.